Wavelength calibration for refractometers

ABSTRACT

Calibration device for calibrating an operation wavelength characteristic of a refractometer for determining an information which is indicative for a refractive index of the sample, wherein the calibration device comprises an itself traceably calibratable determination unit, which is preferably at least partially external of the refractometer, which is adapted for determining an information which is indicative for a discrepancy between a pre-givable set-point-operation wavelength characteristic and an actual-operation wavelength characteristic of the refractometer, on whose basis the refractometer is calibratable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of German Patent Application No. 10 2015 106 795.1 filed 30 Apr. 2015, the disclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

Embodiments of the invention relate to a calibration device, a refractometer, an arrangement and a method for wavelength calibrating of a refractometer.

TECHNOLOGICAL BACKGROUND

With a refractometer, a refractive index of a substance can be measured. The refractive index of a substance or sample depends from both, the wavelength of the light and from its temperature. In order to make measuring results comparable, in refractometers it is thus measured at specified wavelengths and temperatures. Deviations from standardized wavelengths or temperatures during measuring lead to measuring errors. In addition, there are measuring errors of the sensor system which are specific for the respectively related measuring principle. An example are mechanical changes which misadjust the optical set-up.

Relevant prior art is disclosed in U.S. Pat. No. 7,030,989, U.S. Pat. No. 7,492,447, U.S. Pat. No. 8,760,640, U.S. Pat. No. 5,729,347 and DE 10 2009 033 979.

SUMMARY

There may be a need for enabling a precise long-term operation of a refractometer.

Subject-matters with the features according to the independent claims are provided. Further embodiments are shown in the dependent claims.

According to an embodiment of the present invention, a calibration device is provided for calibrating an operation wavelength characteristic of a refractometer for determining an information which is indicative for a refractive index of a sample, wherein the calibration device itself comprises a traceably calibratable (and is preferably at least partially external of the refractometer) determination unit, which is adapted for determining an information which is indicative for a discrepancy between a pre-givable set-point-operation wavelength characteristic and an actual-operation wavelength characteristic of the refractometer, on the basis of which information the refractometer is calibratable, and calibrated, respectively.

According to a further embodiment of the present invention, a refractometer is provided for determining an information which is indicative for a refractive index of a sample, wherein the refractometer comprises a wavelength control unit which is configured for cooperating with a calibration device with the above described features, which is at least partially external of the refractometer, wherein by means of the wavelength control unit, based on the information determined by means of the determination unit of the calibration device, the actual-operation wavelength characteristic is traceable in a direction towards the set-point-operation wavelength characteristic (in particular, the actual-operation wavelength characteristic is adjustable or matchable to the set-point-operation wavelength characteristic).

According to a further embodiment of the present invention, an arrangement is provided which comprises a refractometer, in particular with the above described features, for determining an information which is indicative for a refractive index of a sample, and a calibration device, in particular with the above described features, for calibrating an operation wavelength characteristic of the refractometer.

According to another embodiment of the present invention, a method is provided for calibrating an operation wavelength characteristic of a refractometer for determining an information which is indicative for a refractive index of a sample, wherein in the method, by means of a determination unit which itself is traceably calibratable (and is preferably at least partially external of the refractometer), an information is determined which is indicative for a discrepancy between a pre-given set-point-operation wavelength characteristic and an actual-operation wavelength characteristic of the refractometer, on the basis of which information the refractometer is calibratable or calibrated.

In the context of the present application, the term “operation wavelength characteristic” in particular denotes a wavelength, a wavelength range, a spectral characteristic or a distribution function around a central wavelength, at which and in which, respectively, an electromagnetic radiation is located which is used for determining a refractive index of a sample during operating a spectrometer.

In the context of the present application, the term “pre-givable set-point-operation wavelength characteristic” in particular denotes an operation wavelength characteristic for measuring the refractive index by means of the refractometer, as ideally present in a perfectly calibrated refractometer. The set-point-operation wavelength characteristic may in particular denote the measuring at a (or substantially at a) single desired measuring wavelength (or in a certain spectral range around this ideal measuring wavelength).

In the context of the present application, the term “actual-operation wavelength characteristic” in particular denotes an operation wavelength characteristic for measuring the refractive index by means of the refractometer, as it is present in the reality in a refractometer which is not mandatory ideally adjusted any more (for example in a refractometer which was initially ideally adjusted and whose adjustment was lost over time).

The actual-operation wavelength characteristic thus may deviate from the set-point-operation wavelength characteristic, wherein this deviation can be determined by a calibration and can be balanced out entirely or in part by an adjustment.

In the context of the present application, the term “traceably calibratable determination unit” in particular denotes a determination unit which itself is accessible for a re-calibration by a norm-calibrator. In other words, such a determination unit enables to be calibrated itself, for example when it has lost wavelength accuracy (in particular with respect to a wavelength emission and/or a wavelength detection and/or a wavelength monochromatizing). Thereby, it can be ensured that errors by an inaccurate or lost (for example over time) adjustment of the determination unit can be suppressed and balanced out again, respectively. A “determination unit which is at least partially external of the refractometer” denotes such a one which, in operation or even always, comprises at least a sub-portion which is spatially arranged outside of the refractometer. Such a calibration device is especially good and easily, respectively, traceably calibratable, since it can be detached from the refractometer in order to be calibrated by a norm-calibrator. “Traceably calibrate” may in particular denote that there is a chain of certificates by which it is provable that the measuring means and the standards used in a calibration, themselves have been calibrated by correctly calibrated measuring means, and so forth, until a root of a (in particular national) standard (for example PTB) is achieved. Thereby, a calibration is traceable to (for example national) standards.

In the context of the present application, the term “information which is indicative for a discrepancy between operation wavelength characteristics” in particular denotes the qualitative information that the set-point-operation wavelength characteristic and the actual-operation wavelength characteristic deviate from each other, or denotes the quantitative information to which extent the set-point-operation wavelength characteristic and the actual-operation wavelength characteristic deviate from each other, or denotes only the information which actual-operation wavelength characteristic is currently present, such that a comparison with a set-point-operation wavelength characteristic (for example by means of the determination unit itself or by means of another entity, for example by means of a wavelength control unit) is performable.

According to an embodiment of the present invention, a possibility for calibration and, if necessary and if required, respectively, adjustment of the wavelength of refractometers is provided. This may for example be advantageous when due to aging effects, misadjustment of optical components, temperature fluctuations, etc. the operation wavelength characteristics of a refractometer have changed over time. In particular, a device and a method are provided for measuring the wavelength of the light used in refractometers for measuring the refractive index, and for the traceable calibration and adjustment of the wavelength of the refractometer. By providing a determination unit of the calibration device which is configured for determining a possible wavelength discrepancy in the refractometer at least in part outside of the refractometer and separatable from the refractometer, respectively, this determination unit can itself be calibrated by means of a suitable calibration scale. Undesired artifacts which are caused in calibrating by inaccuracies of, in particular, the determination unit, can thereby be reliably avoided. In addition, such a calibration device which is at least in part detachable from a refractometer and is itself separately calibratable, respectively, has the advantage that a multiplicity of different refractometers can be calibrated with it, i.e. that such a calibration device can be utilized as a portable and universal calibration probe.

In the following, additional exemplary embodiments of the calibration device, the refractometer, the arrangement and the method are described.

According to an exemplary embodiment, the determination unit may comprise a calibratable and/or a calibrated, in particular gauged according to a pre-given (in particular legal) standard, spectrometer for determining the actual-operation wavelength characteristic of the refractometer. With such a spectrometer which may be provided outside of the refractometer and separated from the refractometer, respectively, for example an actual-operation wavelength of the refractometer which is misadjusted with respect to a set-point-operation wavelength, can thus be precisely determined. Such a spectrometer is precisely calibratable itself, such that capturing the information as a basis for a subsequent calibrating is also possible in an error-robust manner.

According to an exemplary embodiment, the determination unit may comprise (a) a coupling optics which is adapted to be optically coupled to the refractometer, in order to couple out electromagnetic measuring radiation out from the refractometer into the coupling optics, and (b) a spectrometer (for example a spectrometer of the above described type) for analyzing the coupled out measuring radiation, in order to determine the actual-operation wavelength characteristic of the refractometer. The coupling optics thus may be an optical probe which preferably can be attached onto the refractometer or inserted into the refractometer, in order to extract measuring radiation as basis for determining an actual-operation wavelength or the like.

According to an exemplary embodiment, the coupling optics may be adapted to be optically coupled with a measuring surface of the refractometer for accommodating a sample to be measured (in a measuring operation), in order to couple out electromagnetic measuring radiation out from the refractometer into the coupling optics. A respective embodiment is for example shown in FIG. 5. The measuring surface of the refractometer may be a surface of a reference-solid body (for example a measuring-prism or a measuring-hemisphere) which is accessible from outside and for a user, respectively, onto which surface, according to the purpose, a sample can be applied whose refractive index shall be determined by means of the refractometer. By applying the coupling optics instead of the sample onto the measuring surface, a part of the measuring radiation can be conducted out of the refractometer and branched off, respectively, for an analysis of the actual-operation wavelength characteristic. This especially preferred embodiment enables in an entirely non-invasive manner to determine the actual-operation wavelength characteristic without penetrating into the optically sensitive interior of the refractometer (i.e. in its sensor path). A further misadjustment of an arrangement of optical component elements in the sensor path of the refractometer which is caused by the calibration process can thereby be advantageously avoided.

According to an exemplary embodiment, the calibration device comprises an, in particular refractive index adapting, coupling liquid for promoting the optical coupling between the measuring surface and the coupling optics. Undesired losses of coupled out electromagnetic radiation due to distinct refraction and/or reflection at boundary surfaces between media with strongly different refractive indices between the reference-body and the coupling optics can be suppressed by such a coupling liquid. The coupling liquid should be optically transparent.

According to an exemplary embodiment, the calibration device may comprise an elastic coupling material at the coupling optics for promoting the optical coupling between the measuring surface and the coupling optics. Undesired losses of coupled out electromagnetic radiation due to a distinct refraction and/or a reflection at boundary surfaces between media with strongly different refractive indices between the reference-body and the coupling optics can be suppressed by such an elastic coupling material. The coupling material should be optically transparent and may for example be realized from an elastic or flexible polymer.

According to an exemplary embodiment, the coupling optics may comprise an optical deflecting element, in particular a beam divider or a mirror, in order to couple out electromagnetic measuring radiation out from the sensor path of the refractometer. A respective embodiment is shown in FIG. 6, for example. The coupling out of the electromagnetic measuring radiation from the refractometer can selectively be performed upstream or downstream (with respect to a propagation direction of the electromagnetic measuring radiation) of a measuring surface of a reference-solid body. The optical deflecting element can be temporarily easily retracted in the optical path in the interior of the refractometer, wherein the electromagnetic measuring radiation to be coupled out from the refractometer can be guided via a light conductor, or the like, out from the refractometer and can then be analyzed, for example by means of a traceably calibratable spectrometer, in order to determine the actual-operation wavelength characteristic.

According to an exemplary embodiment, the determination unit may be adapted for determining the actual-operation wavelength characteristic of the refractometer. According to an alternative embodiment, the determination unit can only determine differences between the actual-operation wavelength characteristic and a set-point-operation wavelength characteristic.

According to an exemplary embodiment, the determination unit may comprise a calibrated electromagnetic calibration radiation source for temporarily feeding electromagnetic calibration radiation of the set-point-operation wavelength characteristic into the refractometer, which after interaction with the sample, is detectable by means of an electromagnetic radiation detector by capturing first detector data. During a non-feeding of the electromagnetic calibration radiation into the refractometer, the sample may be irradiatable with the electromagnetic measuring radiation of the actual-operation wavelength characteristic from an electromagnetic measuring radiation source of the refractometer, and the electromagnetic measuring radiation, after interaction with the sample, is detectable by the electromagnetic radiation detector by capturing second detector data. Such an embodiment is shown in FIG. 7. When the refractometer is perfectly adjusted, i.e. the actual-operation wavelength characteristic corresponds to the set-point-operation wavelength characteristic, the first detector data and the second detector data are equal, since the detector cannot distinguish between the ideal wavelength characteristic of the calibration radiation source and the real wavelength characteristic which is identical thereto of the refractometer. If differences between the first detector data and the second detector data are determined, the measuring wavelength of the refractometer can be changed until the differences become smaller or even disappear. Then the refractometer is calibrated again.

According to an exemplary embodiment, the determination unit may comprise a monochromator for temporarily monochromatizing electromagnetic measuring radiation from an electromagnetic measuring radiation source of the refractometer to the set-point-operation wavelength characteristic, wherein the monochromatized electromagnetic measuring radiation, after interaction with the sample, is detectable by an electromagnetic radiation detector by capturing first detector data. During a non-monochromatizing of the electromagnetic measuring radiation, the sample may be irradiatable with the electromagnetic measuring radiation of the actual-operation wavelength characteristic, and the electromagnetic measuring radiation, after interaction with the sample, may be detectable by the electromagnetic radiation detector by capturing second detector data. Such an embodiment is illustrated in FIG. 8. When the refractometer is perfectly calibrated, i.e. the actual-operation wavelength characteristic corresponds to the set-point-operation wavelength characteristic, the first detector data and the second detector data are equal, since the monochromator then does not change the operation wavelength characteristic. If differences between the first detector data and the second detector data are determined, the measuring wavelength of the refractometer can be changed until the differences become sufficiently small or even disappear. Then the refractometer is calibrated again.

According to an exemplary embodiment, the calibration device may further comprise a wavelength control unit for adjusting the refractometer based on the information determined by means of the determination unit, such that the actual-operation wavelength characteristic is changeable in a direction towards the set-point-operation wavelength characteristic (in particular, the actual-operation wavelength characteristic is matched to the set-point-operation wavelength characteristic). The wavelength control unit thus may be optionally implemented as a part of the refractometer and/or as a part of the calibration device. In other cases it is possible that the wavelength control unit takes the discrepancy between the actual-operation wavelength characteristic and the set-point-operation wavelength characteristic as a reason to re-adjust the wavelength characteristic of the refractometer, in order to reduce or even eliminate the discrepancy. This is equivalent with completing the adjustment and, since the discrepancy is measured quantitatively, also equivalent with completing the calibration.

According to an exemplary embodiment, the refractometer may comprise (a) an electromagnetic measuring radiation source, in particular a broad band measuring radiation source, for emitting electromagnetic measuring radiation to the sample applied on a measuring surface and (b) an electromagnetic measuring radiation detector for detecting the electromagnetic measuring radiation after interaction of the sample with the electromagnetic measuring radiation. For example, the electromagnetic measuring radiation source may be a light-emitting diode, for example a broad band light-emitting diode or another optical light source.

According to an exemplary embodiment, the refractometer may comprise a wavelength selective element for selecting an operation wavelength characteristic from the electromagnetic measuring radiation, for example an interference-bandpass filter, at least one exchangeable filter, an adjustable optical element, a prism, a grating, a monochromator and/or a heating unit for heating the electromagnetic radiation source. By correspondingly controlling, moving, tilting, tempering and/or positioning such a wavelength selective element, a certain measuring wavelength or a certain measuring wavelength band provided by the measuring radiation source can be selected and the operation wavelength characteristic of the refractometer can be adjusted correspondingly.

According to an exemplary embodiment, the refractometer may comprise an actuator (for example comprising an electric motor) which is adapted for actuating the wavelength selective element by means of the wavelength control unit, in order to guide the actual-operation wavelength characteristic in the direction towards the set-point-operation wavelength characteristic. Such an actuator may be a movable actuating element which can, for example by an electrical or an optical controlling, bring the wavelength selective element in a position in a pre-givable manner and can therefore select a desired wavelength characteristic.

According to an exemplary embodiment, the refractometer may be configured for determining the information which is indicative for the refractive index of the sample, from a critical angle of total reflection at a boundary surface between the sample on a measuring surface of a reference-solid body and the reference-solid body, in particular a prism or a hemisphere. This measuring principle is illustrated and described in more detail in FIG. 1, for example.

According to an exemplary embodiment, the calibration device can be configured reversibly separable from the refractometer, such that after separating, the calibration device can be utilized for calibrating an operation wavelength characteristic of another refractometer. In this manner, a single calibration device can be utilized for calibrating many different refractometers in a resource-saving manner. The calibration device for example may be utilized as a portable calibration probe which can be attached onto a refractometer (for example a measuring surface thereof) and can report back the information which results from the associated measuring, in order that in the interior of the refractometer an adaption of the operation wavelength characteristic can be carried out for the purposes of calibrating.

According to an exemplary embodiment, the calibration device and the refractometer can be configured such that the refractometer, after separating the calibration device from the refractometer (and after applying a sample to be measured onto the measuring surface of the refractometer), is operable for determining the information which is indicative for a refractive index of a sample. After detaching the calibration device from the refractometer after completing the calibration, only the sample has to be applied to the measuring surface of the refractometer and the measuring of the refractive index of the sample can start. The effort in conjunction with a re-calibrating of the refractometer thus is extremely low.

According to an exemplary embodiment, the calibration device can be integrated in part in the refractometer and in another part can be configured separated or separable from the refractometer. For example, a wavelength control unit for adjusting an operation wavelength characteristic of the refractometer can be configured as a part of the refractometer, whereas a determination unit may be provided for determining discrepancies between the actual-operation wavelength characteristics and the set-point-operation wavelength characteristics, as part of the calibration device.

According to an exemplary embodiment, the refractometer and/or the calibration device may comprise a data transmission unit, in particular a wired and/or a wireless data transmission unit, for transmitting the determined information from the calibration device to the refractometer, by means of which information the actual-operation wavelength characteristic is matchable in the direction towards the set-point-operation wavelength characteristic. The analyzing of the actual-operation wavelength characteristic can then be carried out non-invasive by means of the calibration device which is external of the refractometer, and thus without disturbing the sensitive sensor path in the refractometer. The real adjustment of the measuring wavelength in the refractometer can then be carried out after the corresponding data transfer from the calibration device to the refractometer.

According to an exemplary embodiment, the calibrating of the refractometer may be performed while the refractometer is located in the field at an end-user for operation. By a calibration device which is portable and preferably not intervening in the sensor path of the refractometer, the calibrating can be performed at the user without technical knowledge, without the need to send the complex refractometer to a manufacturer for calibrating.

Alternatively it is also possible that the result of the determination by the determination unit is a mere information output to a user with the invitation to send the refractometer to the manufacturer for adjusting.

According to an exemplary embodiment, when calibrating, the refractometer may be tested upon its wavelength properties without intervening in an optical path of the refractometer, in particular without opening the refractometer. Thereby, the sensitive optical path in the interior of the refractometer is not disturbed by calibrating.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, exemplary embodiments of the present invention are described in detail with reference to the following figures.

FIG. 1 shows a sub-section of a refractometer according to an exemplary embodiment of the invention in an operational state during measuring a sample.

FIG. 2 shows a sub-section of a refractometer and a sub-section of a calibration device according to an exemplary embodiment of the invention in an operational state during coupling out electromagnetic measuring radiation from the refractometer in a coupling optics and from there into a connected light conductor for determining an information as a basis for subsequently calibrating.

FIG. 3 shows a sub-section of a refractometer and a sub-section of a calibration device according to an exemplary embodiment of the invention in an operational state during coupling out electromagnetic measuring radiation from the refractometer into a coupling optics and from there directly into spectrometer for determining the information as basis for subsequently calibrating.

FIG. 4 shows a sub-section of a refractometer and a sub-section of a calibration device according to an exemplary embodiment of the invention in an operational state during coupling out electromagnetic measuring radiation from the refractometer directly into a light conductor for determining the information as basis for subsequently calibrating.

FIG. 5 shows a refractometer with the calibration device according to an exemplary embodiment of the invention.

FIG. 6 shows a refractometer with the calibration device according to another exemplary embodiment of the invention.

FIG. 7 shows a refractometer with the calibration device according to yet another exemplary embodiment of the invention.

FIG. 8 shows a refractometer with the calibration device according to a further exemplary embodiment of the invention.

Same or similar components in different figures are provided with same reference signs.

Before, referring to the figures, exemplary embodiments of the invention are described, some general aspects of the invention and the underlying technologies shall be explained:

Embodiments of the invention suppress errors in the refractive index-measuring by means of a refractometer by determining errors in the measuring wavelength in the context of a calibration, and if appropriate and necessary, respectively, reducing by adjustment. Exemplary embodiments of the invention thus relate to a solution for calibration and adjustment of the measuring wavelength in a refractometer. In order to better understand the advantageous effects of exemplary embodiments of the invention, it is useful to consider the effects of different error types.

The refractive index measured by a refractometer is determined by the sample, but in addition by diverse error types: ñ(λ,T,S)=n(λ₀ ,T ₀)+(dn/dλ)(λ−λ₀)+(dn/dT)(T−T ₀)+Δn(S)  (1) ñ(λ, T, S) is the direct measuring value of the refractometer at the current wavelength λ of the refractometer, the current sample temperature T and the measuring errors Δn(S) caused by the current state of the sensor system S. All these parameters may change over time, therefore a recurring calibration and, if necessary, adjustment of refractometers is advantageous in order to ensure their accuracy.

It is searched for the refractive index of the substance n(λ₀, T₀) at the standardized wavelength λ₀ and the standard temperature T₀.

Refractometers can be calibrated by measuring reference substances with a known refractive index n(λ₀, T₀) and, if necessary, can be re-adjusted correspondingly. For this purpose, reference substances which are traceable to (for example national) standards can be utilized, and in addition standards are directly defined by some national metrological institutes (for example NIST, PTB).

A frequent expectation of the user is that a use of such traceable refractive index-standards also leads to a traceable calibration or adjustment of the refractometer. In the following, it is examined under which preconditions this expectation is fulfilled.

In the calibration, the deviation between the known reference value and the measuring value is determined: Δn=ñ(λ,T,S)−n(λ₀ ,T ₀)  (2)

In a subsequent adjustment, the such determined deviation is stored in the refractometer and in the following measurements is automatically subtracted from the measuring value by the adjusted refractometer, in order to output the correct value: n(λ₀ ,T ₀)=ñ(λ,T,S)−Δn  (3)

It is implicitly assumed that the deviation Δn is a pure device property of the refractometer and thus can be compensated independently from the sample by an adjustment according to equation (3).

However, equation (1) shows that with a temperature error, also the temperature coefficient of the sample dn/dT contributes to the entire deviation, and with a wavelength error, the wavelength dispersion of the sample dn/dA contributes to the entire deviation: Δn=Δn(S)+(dn/dλ)(λ−λ₀)+(dn/dT)(T−T ₀)  (4)

Thus, it becomes clear that a deviation determined in a calibration with a reference substance can only be used for the adjustment of measurements of other substances with deviating temperature coefficient dn/dT and/or deviating wavelength dispersion dn/dλ, if the error of the temperature and the wavelength is small enough. In other words, small temperature errors and wavelength errors, respectively, are a precondition to be able to reasonably perform the described calibration and adjustment by refractive index-standards. How small the errors have to be depends on the required entire accuracy of the refractometer and on the difference of the samples to be measured.

Many refractometers only measure at one wavelength. As a standard wavelength or set-point-wavelength, 589.3 nm is frequently used.

The wavelength is typically generated by broad band light sources, such as light-emitting diodes (LEDs) whose spectral range is, depending on accuracy requirements, narrowed by wavelength selective elements. For this purpose, interference-bandpass filters can be used, for example. In simple refractometers with moderate accuracy requirements, also LEDs without interference filter can be utilized. In refractometers which are able to measure at multiple wavelengths, for example exchangeable filter or adjustable elements, such as monochromators, can be utilized. In precision refractometers, the accuracy of interference filters in part is not sufficient due to production variance of the central wavelength. Its central wavelength then can be adjusted by tilting the interference filter to the standard wavelength.

In conventional refractometers, the wavelength can be specified in the manufacturing of the refractometer by selection or adjustment of wavelength selective elements.

Wavelength errors may occur when the initial selection or adjustment of the wavelength elective elements is not performed accurately enough. If, for example, LEDs are utilized without further filtering, the production variance of the LEDs may lead to deviating wavelengths. In interference filters, the wavelength is also subjected to an individual variance. When matching adjustable elements, errors may occur.

Further errors may occur when the wavelength of the determining elements changes over time by aging or misadjustment.

Furthermore, errors may occur when wavelength selective elements comprise a temperature dependency and the ambient temperature of the refractometer during a measurement differs from the temperature at the time of selection or adjustment. In particular LEDs are to be mentioned whose emission spectrum is not restricted by further wavelength selective elements, since in particular yellows LEDs comprise a significant temperature range. Wavelength errors at the present cannot be easily recognized or avoided by the user of the devices, since the wavelength of refractometers in the field is not traceably measurable and thus is not calibratable and validatable, respectively, as well. Due to this lack, no refractometers exist so far whose wavelength can be adjusted after manufacturing.

As explained above, for a traceable calibration and adjustment of the refractive index, also a sufficiently small temperature error and thus a traceable calibration of the temperature measuring is advantageously.

An advantageous aspect of an embodiment of the invention is the traceability of a calibration or adjustment to (for example national) standards, in order to comply with modern quality standards. This can be achieved due to the fact that the measuring devices or standards which are utilized in a calibration or adjustment itself are traceable to national standards.

This results in that a wavelength measuring device which is entirely and firmly mounted in a refractometer, which is not calibratable itself, is not suitable for traceable calibrating or adjusting the wavelength of the refractometer. In this case, namely the question would arise how this wavelength measuring device itself is traceably calibrated. By this measure, the problem would only be transmitted from the originally firmly mounted wavelength selective element of the refractometer to the firmly mounted wavelength measuring device.

A solution of the above mentioned problems according to an exemplary embodiment of the invention is to provide a calibration device by which refractometers can be tested upon their wavelength accuracy in an easy manner, namely according certain exemplary embodiments of the invention especially advantageous without the need to open the refractometers or intervene in them in another way. This enables to measure, thus to calibrate, the wavelength of the light used by the refractometer for refractive index measuring from the outside by means of a traceably calibratable measuring device.

Due to the simple traceable calibration of the wavelength with the aid of exemplary embodiments of the invention, it also becomes practicable to equip refractometers with an adjustment possibility of the wavelength, such that a wavelength error determined in the measuring can be eliminated by adjustment also after manufacturing. In this way, the wavelength of the refractometer can also be traceably adjusted.

Detailed Description of Illustrated Embodiments

FIG. 1 shows a sub-section of a refractometer 1 according to an exemplary embodiment of the invention in an operational state during measuring a sample 19.

This refractometer 1 determines the refractive index of the sample 19 from the critical angle of total reflection at a boundary surface (see measuring surface 24) to the reference-solid body 2.

Conventionally fixedly mounted interference filter for a stabilizing of the wavelength in refractometers are known (not shown).

The reference-solid body 2 can be implemented as prim (also referred to as measuring prim) or as hemisphere, for example. Such an arrangement with the prism is illustrated in FIG. 1. When the sample 19 is applied onto the measuring surface 24 of the refractometer 1, during irradiating electromagnetic measuring radiation 6 originating from the electromagnetic measuring source 3, such light (see transmitted electromagnetic radiation 26) with an intensity I is refracted in the sample 19, which hits the measuring surface 24 at a steeper angle α than it corresponds to the critical angle α_(G) of total reflection (see diagram 50 in FIG. 1). Characteristic measuring ranges of refractometers 1 are limited at small refractive indices. This leads to the fact that in a refractometer 1 without sample 19, normally no measuring light is transmitted to the outside. Reflected electromagnetic measuring radiation is denoted with reference sign 7 in FIG. 1 and hits a position sensitive electromagnetic measuring radiation detector 8. Reference sign 4 shows a lens.

FIG. 2 shows a sub-section of an arrangement 20 with a sub-section of a refractometer 1 and a sub-section of a calibration device 16 according to an exemplary embodiment of the invention in an operational state during an coupling out of electromagnetic measuring radiation from the refractometer 1 into a coupling optics 15 and from there into a connected light conductor 12 for determining an information as a basis for a subsequently calibrating.

An embodiment of the invention implements the calibration device 16 which enables to measure the wavelength of the light used by the refractometer 1 for refractive index-measuring from the outside. A principle realized according to exemplary embodiment of the invention (see for example FIG. 2, FIG. 3 and FIG. 4) is that, without the sample 19, a coupling optics 15 is attached on the measuring surface 24 of the refractometer 1, in order to conduct measuring light from the refractometer 1 and subsequently being enabled to analyze the measuring light, for example in a calibrated spectrometer 13 (not illustrated in FIG. 2, see for example FIG. 3).

For this purpose, it is advantageous to bring the coupling optics 15 in optical contact with the measuring surface 24 of the refractometer 1, i.e. to optically couple them, such that electromagnetic radiation can be transmitted from the reference-solid body 2 through the measuring surface 24 into the coupling optics 15 for further wavelength analysis. It is helpful for this purpose to provide a coupling fluid (not shown) for a better optically coupling between the measuring surface 24 and the coupling optics 15.

The coupling optics 15 can be optically connected with the measuring prism as reference-solid body 15 with the aid of a coupling liquid (not shown, “index matching fluid”, for example immersion oil). Further, also an optical layer (for example made of an elastomer, not shown) can be attached to the coupling optics 15, in order to achieve an effective coupling to the measuring prism as reference-solid body 2.

According to FIG. 2, the coupling optics 15 as coupling element also serves for coupling out the light from the refractometer 1. The light conductor 12 attached thereto with a suitable angle guides the light to a spectrometer 13 (not shown in FIG. 2, see reference sign 13 in FIG. 3), with whose aid the wavelength of the measuring light is measured.

FIG. 3 shows a sub-section of a refractometer 1 and a sub-section of a calibration device 16 according to another exemplary embodiment of the invention in an operational state during coupling out electromagnetic measuring radiation from the refractometer 1 into the coupling optics 15 and from there directly into the spectrometer 13 for determining the information as basis for subsequently calibrating. According to FIG. 3, the coupling optics 15 is implemented such that it directs the coupled out light without the light conductor 12 directly into the spectrometer 13.

FIG. 4 shows a sub-section of a refractometer 1 and a sub-section of a calibration device 16 according to an exemplary embodiment of the invention in an operational state during coupling out electromagnetic measuring radiation from the refractometer 1 directly into a light conductor 12 for determining the information as basis for subsequently calibrating. According to FIG. 4, the separate coupling body 15 according to FIG. 2 and FIG. 3 is thus omitted and implemented by the light conductor 12 itself, respectively. The coupling is thus performed directly into the light conductor 12 which is preferably grinded in an inclined manner which conducts the coupled out light into a spectrometer (not shown in FIG. 4, see reference sign 13 in FIG. 3).

According to an exemplary embodiment of the invention, the above described calibration device 16 is used for adjusting the wavelength of the respective refractometer 1. For this purpose, the refractometer 1 can be constructed such that its wavelength can be changed by an adjustable wavelength selective element (see reference sign 9 in FIG. 5 to FIG. 8), also after manufacturing the refractometer 1.

FIG. 5 shows an arrangement 20 of a refractometer 1 and a calibration device 16 according to a preferred exemplary embodiment of the invention.

More precisely, the refractometer 1 is configured for determining an information which is indicative for the value of the refractive index of a sample 19 (not shown in FIG. 5 and not provided according to the calibration mode according to FIG. 5), when the refractometer is operated in a measuring mode (see for example FIG. 1). The measuring principle of the refractometer 1 for measuring a refractive index of a sample 19 is based on determining the information which is indicative for the refractive index of the sample 19 from a critical angle of total reflection at a boundary surface between the sample 19 on the measuring surface 24 of the reference-solid body 2 and the reference-solid body 2 (see also FIG. 1).

In addition, the calibration device 16 is configured for cooperating with the refractometer 1 for calibrating an operation wavelength characteristic of the refractometer 1 in the calibration mode illustrated in FIG. 5. The calibration device 16 comprises a determination unit 17 which itself is traceably calibratable and which is external of the refractometer (i.e. provided outside of the refractometer 1), which determination unit 17 is adapted for determining the information which is indicative for a discrepancy between a pre-givable set-point-operation wavelength characteristic of the refractometer 1 and an actual-operation wavelength characteristic of the refractometer 1. On the basis of this externally captured information, the refractometer 1 can be subsequently calibrated. In more detail, the determination unit 17 according to FIG. 5 is intended for determining the actual-operation wavelength characteristic of the refractometer 1, i.e. a current measuring wavelength of the refractometer 1. The exact function principle of the determination unit 17 is described in more detail below. Due to the fact that the determination unit 17 is arranged outside of the refractometer 1 and is separated and detachable from the refractometer 1, respectively, the determination unit 17 for determining the current measuring wavelength is calibratable itself by a calibrator which is not shown, and is therefore not a noteworthy error source itself when calibrating the refractometer 1.

The refractometer 1 comprises a wavelength control unit 11 which is configured for cooperating with the calibration device 16 which is external of the refractometer. By means of the wavelength control unit 11, based on the information determined by the determination unit 17 of the calibration device 16, the actual-operation wavelength characteristic can be guided back in the direction towards the set-point-operation wavelength characteristic, consequently the refractometer 1 is wavelength-adjusted. In other words, when the determination unit 17 determines, that the current, real measuring wavelength deviates from a target-measuring wavelength or set-point-measuring wavelength at which the refractometer 1 should work according to a pre-given specification, the wavelength control unit 11 can change the measuring wavelength in the manner described below in more detail, in order to harmonize the real measuring wavelength with the target-measuring wavelength.

Firstly, the set-up of the refractometer 1 is described in more detail. The refractometer 1 contains the electromagnetic measuring radiation source 3 in form of a broad band light emitting diode for emitting light as electromagnetic measuring radiation 6. The electromagnetic measuring radiation 6 propagates through the lens 4 and a controllable wavelength selective element 9 for selecting a certain measuring wavelength from the broad band spectrum of the electromagnetic measuring radiation source 3 to another lens 5 and from there to the sample 19 which is applied on the measuring surface 24 of a reference-solid body 2 in a regular measuring operation, whose refractive index is to be determined. However, FIG. 5 shows the arrangement 20 in an operational state in which no sample is applied on the measuring surface 24, since in the shown operational state the refractometer 1 shall be calibrated. Thus, on the measuring surface 24 in the calibration operation, a coupling body 15 is attached as external probe.

Electromagnetic measuring radiation 7 reflected in the region of the measuring surface 24 propagates through an optional lens 29 to the electromagnetic measuring radiation detector 8 for detecting the electromagnetic measuring radiation 7. The measuring radiation detector 8 is a position sensitive detector, for example a one-dimensional or two-dimensional array of light sensitive elements.

The wavelength selective element 9 serves for selecting an operation wavelength from the electromagnetic measuring radiation 6 and may be configured as an interference-bandpass filter, for example. The refractometer 1 further contains an actuator 10 which is adapted for actuating the wavelength selective element 9 by means of the wavelength control unit 11, in order to perform a desired wavelength change, in particular in order to match an actual-operation wavelength which is misadjusted in an undesired manner with respect to an adjusted state, in the direction towards the set-point operation wavelength. As illustrated in FIG. 5, the wavelength control unit 11 controls, in addition to the actuator 10, also the electromagnetic measuring source 3 and contains the detector data from the measuring radiation detector 8. By means of the wavelength control unit 11 it is further possible to control the temperature course of the electromagnetic measuring source 3 and thus the emitted wavelength of the electromagnetic measuring source 3.

In the following, the calibration device 16 is described in more detail. The determination unit 17 of the calibration device 16 comprises a spectrometer 13 which is calibrated according to a pre-given standard, for determining the actual-operation wavelength of the refractometer 1. In addition, the determination unit 17 contains the coupling optics 15 which is adapted to be optically coupled with the refractometer 1, in order to couple out electromagnetic measuring radiation from the refractometer 1 into the coupling optics 15. The spectrometer 13 receives via a light conductor 12 the coupled out measuring radiation from the coupling optics 15 for analyzing the same, in order to determine the actual-operation wavelength of the refractometer 1. For this purpose, the coupling optics 15 is attached on the measuring surface 24 (which in the normal measuring operation actually serves for accommodating the sample 19) of the refractometer 1 and thereby optically coupled. Optionally, a refractive index adapting coupling liquid (not shown) or an elastically coupling material (not shown) can be utilized at the coupling optics 15 for promoting the optical coupling between the measuring surface 24 and the coupling optics 15.

Advantageously, according to FIG. 5, the entire calibration device 16 is configured such that it is separable from the refractometer 1, such that after separating the calibration device 16 is utilizable for calibrating an operation wavelength of another refractometer (not shown in the figure). The refractometer 1 according to FIG. 5, after separating the calibration device 16 from the refractometer 1, is operable for determining a refractive index of a sample 19 which is to be applied on the measuring surface 24 for this purpose. The calibration device 16 further comprises a wired or a wireless data transmission unit 14 for transmitting the determined information about the actual-wavelength from the determination unit 17 of the calibration device 16 to the wavelength control unit 11 of the refractometer 1. With this information, the wavelength control unit 11 can adjust the transmitted actual-operation wavelength in the direction towards the set-point operation wavelength which may be pre-known to the wavelength control unit 11. For this purpose, the wavelength control unit 11 can correspondingly control the actuator 10, in order to move the wavelength selective element 9, in order to change the measuring wavelength arriving at the reference-solid body 2 and thereby adjust the refractometer 1, respectively. Thus, according to FIG. 5, the wavelength of the measuring light (electromagnetic measuring radiation 6) of the refractometer 1 is changeable by tilting an interference-bandpass filter as wavelength selective element 9. The tilting and thereby the wavelength is adjusted by the actuator 10 of the control unit in form of the wavelength control unit 11. The adjustment is performed by measuring the actual-operation wavelength of the refractometer 1 by the above described calibration device 16 and by transmitting this value through a wired or wireless connection (in form of the data transmission unit 14) into the control unit/wavelength control unit 11 of the refractometer 1. The control unit/wavelength control unit 11 adjusts, with the aid of the actuator 10, the wavelength selective element 9, such that the measured wavelength corresponds to the pre-given wavelength. Alternatively, the calibration device 16 can firstly send the measured wavelength to a computer or processor (not shown) which controls the adjustment process and sends the desired wavelength adjustment to the control unit/wavelength control unit 11 in the refractometer 1.

For this method, alternatively another adjustable wavelength selective element 9 can be utilized, for example a prism, a grating (in particular a holographic grating) etc. In addition to tilting the interference filter for adjustment of the center of gravity wavelength, there is also the possibility of an insertable, linear-variable filter bandpass filter or an edge filter or a liquid crystalline bandpass filter or also an electronically adjustable filter module.

Alternatively, the wavelength control unit 11 can adjust the intended set-point operation wavelength by means of a temperature calibration curve which is stored in a storage element.

FIG. 6 shows a refractometer 1 with a calibration device 16 according to another exemplary embodiment of the invention. According to FIG. 6, the coupling optics 15 comprises an optical deflection element 25 (for example a beam divider) in order to couple electromagnetic measuring radiation 6 out from the refractometer 1. According to FIG. 6, this is performed upstream of the measuring surface 24, but it may also be performed downstream of the measuring surface 24.

Thus, alternatively to FIG. 5, a coupling body in form of the coupling optics 15 can also be integrated in the sensor system of the refractometer 1, as shown in FIG. 6. Either between the electromagnetic measuring source 3 and the reference measuring body 2 which is configured as measuring-prism here, or between the reference-measuring body 2 and the measuring radiation detector 8, an optical coupling element/a coupling optics 15 which is here configured as beam divider with the light conductor 12 can be mounted, in order to guide measuring light from the sensor system and, after its characterizing, subsequently calibrate the wavelength of the measuring light. Thus, according to FIG. 6, measuring light is guided out from the refractometer 1, in order to measure the measuring wavelength which is defined by the electromagnetic measuring source 3 in combination with the wavelength selective element 9 by a pre-calibrated and/or certified spectrometer 13. Alternatively or in addition to a standardized spectrometer, also a Michelson-lambdameter etc. can be used.

FIG. 7 shows a refractometer 1 with a calibration device 16 according to yet another exemplary embodiment of the intervention. In contrast to FIG. 5 and FIG. 6, according to FIG. 7, now a sample 19 (for example at test sample with pre-known properties) is applied to the measuring surface 24 of the refractometer 1.

According to FIG. 7, the determination unit 17 which is external of the refractometer comprises an adjusted (and again traceably calibratable at any time and adjustable, if necessary) electromagnetic calibration radiation source 21 for temporarily feeding the electromagnetic calibration radiation of the set-point-operation wavelength into the refractometer 1. By means of the calibration radiation source 21 and a connected light conductor 12, a mirror 30 which is introduced in the sensor path, in particular a partially transmissive mirror is irradiated with the electromagnetic calibration radiation, from where the electromagnetic calibration radiation is reflected to the sample 19 and, after interaction with the sample 19, is detected by the electromagnetic measuring radiation detector 8 by capturing first detector data. In other periods of time, in which, instead of feeding the electromagnetic calibration radiation into the refractometer 1, the same sample 19 is irradiated with electromagnetic measuring radiation 6 of the actual-operation wavelength from the electromagnetic measuring radiation source 3 of the refractometer 1, the electromagnetic measuring radiation 6, after interaction with the sample 19, reaches the electromagnetic measuring radiation detector 8 which on this occasion detects the second detector data. If the first detector data are different from the second detector data, then the real current actual-measuring wavelength is detuned with respect to a set-point-measuring wavelength and can be adjusted by matching to the set-point-measuring wavelength. For this purpose, the wavelength control unit 11 which is supplied with the first and second detector data may control the actuator 10 for changing the position of the wavelength selective element 9, whereby from the broadband spectrum of the electromagnetic measuring radiation source 3 which is configured as light emitting diode, another corrected measuring wavelength is selected. Alternatively to this, the wavelength control unit 11 can be configured such that it is enabled to control the temperature course of the electromagnetic measuring source 3 and thus adjusts the actual-operation wavelength.

FIG. 7 thus shows an embodiment of the invention which is alternative to FIG. 5 and FIG. 6, with an externally calibrated light source in form of the calibration radiation source 21 (for example a Na-spectral lamp) which sends out a wavelength λ0. The light with the calibrated wavelength λ0 is fed into the sensor path, for which purpose the mirror 30 and the light conductor 12 are provided. A test sample as a sample 19 is measured (n1λ) on the prism. Subsequently, the calibrated light source, i.e. the calibration radiation source 21, is switched off or hidden. Then, by means of the electromagnetic measuring radiation source 3, i.e. the light source of the refractometer 1, the same test sample is measured (n2λ) on the prism and the interference filter as wavelength selective element 9 is adjusted until both refractive index measurements correspond to each of the (n1=n2).

The calibrated external light source, i.e. the calibration radiation source 21 can also be realized by a calibrated monochromator plus an arbitrary light source or can be realized by an arbitrary adjustable monochromator and a light source which can also be measured via a calibrated spectrometer.

FIG. 8 shows a refractometer 1 with a calibration device 16 according to a further exemplary embodiment of the invention.

According to FIG. 8, the determination unit 17 comprises a monochromator 22 for temporarily monochromatizing electromagnetic measuring radiation from an electromagnetic measuring radiation source 3 of the refractometer 1 to the set-point-operation wavelength. The monochromatized electromagnetic measuring radiation, after interaction with the sample 19, is detected by an electromagnetic radiation detector 8 by capturing first detector data. In other periods of time, in which no monochromatizing of the electromagnetic measuring radiation is performed, the same sample 19 is irradiated with the electromagnetic measuring radiation of the actual-operation wavelength. Then, the electromagnetic measuring radiation, after the interaction with the sample 19, is detected by the electromagnetic radiation detector 8 by capturing second detector data. From the difference between the first detector data and the second detector data, the extent of a required wavelength adjustment can be determined. If, in contrast, the first detector data and the second detector data correspond to each other, no adjustment is required and the refractometer 1 is calibrated, respectively.

FIG. 8 thus describes an alternative embodiment to the wavelength calibration from FIG. 7. The light from the electromagnetic measuring radiation source 3 is deflected (for this purpose, a shutter can be folded in), conducted via the mirror 32 into a calibrated or calibratable monochromator 21, and fed back via a further mirror 33 into the sensor path. The refractive index of the test sample (see reference sign 19, which lies on the measuring surface 24 of the prism as reference-solid body 2) n1 is measured. Similar to FIG. 7, with a folded out shutter 31, the interference filter as wavelength selective element 9 is adjusted until n2=n1.

The test sample according to FIG. 7 and FIG. 8 may for example be a sample which comprises a wavelength dispersion (dn/dλ*0), for example water.

A further exemplary embodiment of the invention is suitable in particular for refractometers 1 in which the wavelength is mainly determined by the center of gravity wavelength of an electromagnetic measuring radiation source 3. In this case, the wavelength can be adjusted by changing the temperature of the electromagnetic radiation source 3 (for example a laser diode). An adjustment of the wavelength corresponding to an embodiment of the invention is achieved by bringing the electromagnetic radiation source 3 to a suitable temperature by means of a tempering unit, such that the desired wavelength is achieved.

Complementary, it should be noted that “comprising” does not exclude other elements or steps and “a” or “an” does not exclude a multiplicity. Further it should be noted that features or steps which are described with reference to one of the above embodiments can also be used in combination with other features or steps of other embodiments described above. Reference signs in the claims shall not be considered as limitation. 

The invention claimed is:
 1. A calibration device for calibrating an operation wavelength characteristic of a refractometer for determining information indicative for a refractive index of a sample, the calibration device comprising: a determination unit which itself is traceably calibratable and which is at least partially external of the refractometer, which determination unit determines information indicative for a discrepancy between a pre-givable set-point-operation wavelength characteristic and an actual-operation wavelength characteristic of the refractometer, on the basis of which information the refractometer is calibratable.
 2. The calibration device according to claim 1, wherein the determination unit comprises: a calibratable and/or a calibrated, according to a pre-given standard, spectrometer for determining the actual-operation wavelength characteristic of the refractometer.
 3. The calibration device according to claim 1, wherein the determination unit comprises: a coupling optics which is adapted to be optically coupled with the refractometer, in order to couple electromagnetic measuring radiation out of the refractometer into the coupling optics; and a spectrometer for analyzing the coupled out measuring radiation, in order to determine the actual-operation wavelength characteristic of the refractometer.
 4. The calibration device according to claim 3, wherein the coupling optics is adapted to be optically coupled with a measuring surface of the refractometer for accommodating a sample to be measured, in order to couple electromagnetic measuring radiation out from the refractometer into the coupling optics; wherein the calibration device comprises at least one of the following features: a refractive index adapting, coupling liquid for promoting the optical coupling between the measuring surface and the coupling optics; an elastic coupling material at the coupling optics for promoting the optical coupling between the measuring surface and the coupling optics.
 5. The calibration device according to claim 3, wherein the coupling optics comprises: an optical deflecting element insertable into a sensor path of the refractometer in order to couple electromagnetic measuring radiation out from the refractometer.
 6. The calibration device according to claim 1, wherein the determination unit is adapted for determining the actual-operation wavelength characteristic of the refractometer.
 7. The calibration device according to claim 1, wherein the determination unit comprises: a calibrated electromagnetic calibration radiation source for temporarily feeding electromagnetic calibration radiation of the set-point-operation wavelength characteristic into the refractometer, which after interaction with a pre-known reference sample, is detectable by means of an electromagnetic radiation detector by capturing first detector data; wherein during a non-feeding of the electromagnetic calibration radiation into the refractometer, the sample is irradiatable with electromagnetic measuring radiation of the actual-operation wavelength characteristic from an electromagnetic measuring radiation source of the refractometer, and the electromagnetic measuring radiation, after interaction with the sample, is detectable by the electromagnetic radiation detector by capturing second detector data.
 8. The calibration device according to claim 1, wherein the determination unit comprises: a monochromator for temporarily monochromatizing electromagnetic measuring radiation from an electromagnetic measuring radiation source of the refractometer to the set-point-operation wavelength characteristic by conducting the electromagnetic measuring radiation through the monochromator in a first operation mode, wherein the monochromatized electromagnetic measuring radiation, after interaction with a pre-known reference sample, is detectable by an electromagnetic radiation detector by capturing first detector data; wherein the electromagnetic measuring radiation is not conducted through the monochromator during a second operation mode in which the sample is irradiatable with the electromagnetic measuring radiation of the actual-operation wavelength characteristic, and the electromagnetic measuring radiation, after interaction with the sample, is detectable by the electromagnetic radiation detector by capturing second detector data.
 9. The calibration device according to claim 1, further comprising: a wavelength control unit for adjusting the refractometer based on the information determined by the determination unit, such that the actual-operation wavelength characteristic is matchable in a direction towards the set-point-operation wavelength characteristic.
 10. A refractometer for determining information indicative for a refractive index of a sample, the refractometer, comprising: a determination unit traceably calibratable and adapted to determine an information which is indicative for a discrepancy between a pre-givable set-point-operation wavelength characteristic and an actual-operation wavelength characteristic of the refractometer, on the basis of which information the refractometer is calibratable; a wavelength control unit configured for cooperating with a calibration device which is at least partially external of the refractometer, wherein by the wavelength control unit, based on the information determined by the determination unit of the calibration device, the actual-operation wavelength characteristic is matchable in the direction towards the set-point-operation wavelength characteristic.
 11. The refractometer according to claim 10, wherein the refractometer further comprises: a broad band electromagnetic measuring radiation source, for emitting optical measuring radiation, to the sample applied on the measuring surface; and an electromagnetic measuring radiation detector for detecting the electromagnetic measuring radiation, after interaction of the sample with the electromagnetic measuring radiation.
 12. The refractometer according to claim 11, wherein the refractometer further comprises: a wavelength selective element from the group consisting of an interference-bandpass filter, at least one changeable filter, an adjustable optical element, a prism, a grating, a monochromator and a heating unit for heating the electromagnetic measuring radiation source.
 13. The refractometer according to claim 12, further comprising: an actuator adapted controllably by the wavelength control unit for actuating the wavelength selective element, in order to adjust the actual-operation wavelength characteristic in the direction towards the set-point operation wavelength characteristic.
 14. The refractometer according to claim 11, further comprising: a wavelength control unit adapted to adjust the actual-operation wavelength characteristic of the electromagnetic measuring radiation source, with the aid of a change of the temperature of the electromagnetic measuring radiation source, in the direction towards the set-point-operation wavelength characteristic.
 15. The refractometer according to claim 10, wherein the refractometer is configured for determining the information indicative for the refractive index of the sample, based on a critical angle of total reflection at a boundary surface between the sample at a measuring surface of a reference-solid body on the one hand and the reference-solid body on the other hand.
 16. An arrangement, comprising: a refractometer, for determining an information indicative for a refractive index of a sample, the refractometer having a wavelength control unit at least partially external of the refractometer, and a calibration device, in communication with the refractometer for calibrating an operation wavelength characteristic of the refractometer, wherein by the wavelength control unit, based on the information determined by a determination unit of the calibration device, the actual-operation wavelength characteristic is matchable in the direction towards the set-point-operation wavelength characteristic.
 17. The arrangement according to claim 16, wherein the calibration device is configured separable from the refractometer, such that after separating, the calibration device is utilizable for calibrating an operation wavelength characteristic of another refractometer; and/or wherein the calibration device and the refractometer are configured such that the refractometer, after separating the calibration device from the refractometer, is operable for determining an information which indicative for a refractive index of a sample.
 18. The arrangement according to claim 16, wherein the calibration device in part is integrated in the refractometer and in another part is configured separately or separatably from the refractometer; and/or wherein the refractometer and/or the calibration device comprises a data transmission unit for transmitting the determined information from the calibration device to the refractometer, by which information the actual-operation wavelength characteristic is matchable in the direction towards the set-point-operation wavelength characteristic.
 19. A method, comprising: calibrating an operation wavelength characteristic of a refractometer with a determination unit which itself is traceably calibratable at least partially external of the refractometer; by determining information indicative for a discrepancy between a pre-given set-point-operation wavelength characteristic and an actual-operation wavelength characteristic of the refractometer, on the basis of which information the refractometer is calibrated.
 20. The method according to claim 19, further comprising: adjusting the refractometer, based on the information indicative for the discrepancy, such that the actual-operation wavelength characteristic is matched in a direction towards the set-point-operation wavelength characteristic; wherein the method comprises at least one of the following features: the calibrating of the refractometer is performed while the refractometer is located in the field at a user for operation; when calibrating, the refractometer is tested upon its wavelength properties without intervening in a sensor path of the refractometer without opening the refractometer. 